The present invention relates to a line selection circuit for selecting a horizontal line within a raster of transmitted video information. More particularly, the invention relates to a line selection circuit for a color television receiver which will select a line during the vertical blanking interval, such line, for example, is one containing reference information for the automatic control of the television receiver.
U.S. Pat. No. 3,780,218 issued Dec. 18, 1973 to John L. Rennick describes a circuit for automatically setting the hue in a color television receiver by utilizing a vertical interval reference signal (VIR signal) contained in one of the horizontal lines in the received video information. The Rennick patent describes a circuit for identifying the line containing the VIR signal to enable circuitry in the receiver to respond to the VIR signal.
Since no recognizable information is transmitted to identify a particular line, such as the line containing a VIR signal, and since many lines could contain reference information for different purposes, it is necessary, as the aforementioned Rennick patent indicates, to provide a line selecting and identifying circuit for a television receiver if automatic control by means of a reference signal contained in a particular line is to be achieved.
The line selection circuit shown by the Rennick patent employs digital circuitry practical only with the use of integrated circuits. However, since the use of an approach solely dependent upon integrated circuits may be undesirable either due to cost or timing, it is accordingly an object of the present invention to provide a line selection circuit adaptable to fabrication with discrete components or integrated circuits.
It is another object of the present invention to provide a line selection circuit that is not frustrated by the interlace between the fields in each frame of the televised signal.
Another object of the present invention is to provide a line selection circuit in which horizontal sync information is utilized to initiate and terminate the timing period between a first line and the desired line, thereby avoiding the need for precise timing circuitry.
These and other objects are achieved by the present invention by the provision of integration means responsive to the sync information in the received video signal to emphasize the vertical sync information in each field relative to the other sync information thereby forming a vertical sync pulse for each field. Since the vertical sync pulse for the even field differs in position from the vertical sync pulse of the odd field relative to the line numbers by one-half line (necessary for the interlace of the two fields), this pulse alone is not sufficient to initiate the timing period for the selection of the desired line. This problem is overcome by adding horizontal sync information to the vertical sync information to derive a composite sync pulse for each field. The composite pulse is utilized to trigger first threshold means contained in a first one-shot multivibrator. The composite pulse triggers the one-shot to change its state of conduction on the same line in each field, thereby overcoming the interlace which would otherwise cause the initiation of timing period to be a half line different on successive fields. The one-shot is selected to have a timing period that would nominally terminate within the desired line. Precise termination of the timing period is achieved by adding horizontal sync information to the normal timing pulse of the one-shot so that the timing period is sharply terminated at the beginning of the desired line. In this manner, the requirements for precise termination of the timing period that would ordinarily be placed on the multivibrator are avoided and an ordinary low tolerance device can be employed. A second one-shot multivibrator responding to the termination of the timing period may also be used to generate an activation pulse to activate circuitry to respond to the information contained on the predetermined line, the activation pulse lasting for the duration of the predetermined line.